Two postoperative complications which may occur after repair of coarctation of the aorta are hypertensive crisis and mesenteric vasculitis. The etiology and mechanism of these potential hazardous complications, which may be related to sympathetic rebound phenomena, is difficult to establish and investigate because of the difficulties in developing an animal model and determining regional blood flow. A model of coarctation of the aorta has been developed by constricting the descending thoracic aorta with tape tied over felt pads. The animal models were allowed to mature, each retaining a significant gradient across the coarctation. In these animals regional blood flow distribution will be determined (by radioactive microspheres) to myocardium, kidney, GI tract, and skeletal muscle above and below the coarctation before and after surgical repair of the coarctation in the awake state. Also, coincident with these blood flow measurements, serum epinephrine and norepinephrine levels will be obtained for determination and used as an index of sympathetic tone. It is hoped that this study will provide some insight into the etiology and mechanism of the postoperative complications that occur with surgical correction of coarctation of the aorta.